US 3474254 A
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Oct. 21, 1969 w. PIEPENBRINK ETAL 3,474,254
PHOTOELECTRONIC APPARATUS FOR SCANNING TEXTILE MATERIAL Filed Feb. 26, 1968 United States Patent 3,474,254 PHOTOELECTRONIC APPARATUS FOR SCANNING TEXTILE MATERIAL Winfried Piepenbrink and Adolf Triller, Munich, Germany, assignors to Erwin Sick, Waldkirch im Breisgau,
Germany Filed Feb. 26, 1968, Ser. No. 708,209 Int. Cl. G01n 2.1/30, 21/16, 21/32 US. Cl. 250--219 Claims ABSTRACT OF THE DISCLOSURE BRIEF SUMMARY OF THE INVENTION This invention relates to a photoelectronic arrangement for scanning material strips, particularly strips of textile fabric, for the purpose of determining defects in the fabric caused, for example by the breaking of a thread or of a needle. For the purpose of such detection in plain woven material it is known to periodically move an optical head transversely above the material, said head comprising a lamp and a photoelectronic receiver. Upon occurrence of a defect caused, for instance, by the breaking of a thread or by a defective needle, a dark longitudinal line occurs at this point of the material which line is detected by the photoelectronic arrangement.
The afore-described method of detecting defects, however, is not operative when the fabric is made of threads of different colors, for example when it is constituted as a white material having dark longitudinal lines. In such cases, the contrast of the darker threads with respect to the brighter base material may then be a multiple of the defect contrast. It will then be impossible for the photoelectronic arrangement to distinguish the intended black lines from defect lines.
It has already been proposed to utilize for the purpose of detecting defects, an increased light transmitting quality of the material at the defective point. This may be effected by placing, at the scanning point, a highly refleeting material underneath the material to be examined and that that portion of the light reflected through the defect is utilized to trigger the defect signal. However, this method can only be applied successfully with large defects causing a broad gap in the fabric. With many kinds of textile fabric, a defective region allows only very little light to pass through it, so that here, too a reliable discrimination between defect and pattern is impossible.
The invention is based on the known utilization of socalled back reflectors constituted by reflecting material underneath the strip of textile fabric. It is known that back reflectors reflect the light directed onto them only in the direction of incidence. The surface of the material to be examined however, may be regarded as a diffuse reflector uniformly reflecting the light directed thereon in the solid angle 21r. With relatively compact material, the diffuse radiation reflected in the optical head is dominant and accordingly the signal is modulated by the dark or bright lines in the material.
According to the invention, this undesired modulation is eliminated by the use of two bridge connected photoelectronic transducers.
The invention contemplates a photoelectronic defect 3,474,254 Patented Oct. 21, 1969 monitoring apparatus, particularly for monitoring woven material strips including a light sensitive scanning device for scanning the material strip in a transverse direction and comprising a back reflector placed underneath the material strip to be monitored. The monitoring arrangement is characterized, according to the invention, by the provision of two light detectors the outputs of which are connected in a bridge or differential connection, one of said light detectors being responsive only to the diffuse light reflected by the material strip, the second light de tector being responsive, with the same sensitivity, as the first light detector to the light diffusely reflected by the material strip and further being arranged in autocollimation with respect to the light source, and thereby also responsive to the light reflected by the back reflector.
When the material is without defect, the same diffusely reflecting radiation is directed onto both light detectors which are photoelectronic transducers and no signal occurs at the output of the bridge connection. In case of a defect, the transducer in the autocollimation radiation path additionally receives the light passing through the defect and again reflected back, and the bridge is correspondingly tuned out and generates a defect signal.
With glossy material consisting of threads of different thickness and possibly different cross-section, it is possible that because of the different angles of observation of the two receivers, a difference in signal occurs even with faultless material because of a radiation directionally reflected by the material. This may be prevented by an arrangement in which both receivers observe the material at the same angle and wherein the separation of material reflection and reflector radiation is effected in a non geometrical manner. According to a further aspect of the invention, this is effected by the use of polarization filters. The vibration direction of the light reflected by the back reflector, arranged underneath the material strip to be monitored is not changed thereby, whereas the radiation reflected by the material is dispersed and the polarization nullified.
Preferably, the arrangement is designed such that both light detectors are arranged in autocollimation with respect to the light source and the light source emits polarized light, and at least before one light detector there is arranged a polarizing device which polarizes perpendicularly to the polarization device of the light source.
However, it is also possible to provide a polarization device before each light detector, the polarization devices polarizing perpendicularly with respect to each other.
If a polarization device is arranged only before one light detector, a means is employed through which the output signal of that light detector before which no polarization device is arranged, upon admittance with stray light, occurs with the same intensity as the signal supplied by the other light detector before which there is arranged the polarization device. Polarization devices polarizing perpendicularly to one another may also be provided before both light detectors, whereby it is insured that both light detectors respond with the same intensity to the diffusely reflected light.
The light reflected by the back reflector may be accordingly received by one transducer only, whereas the light reflected by the material being examined is received by both transducers. Thus a tuning-out of the bridge is effected only by the light passing through a defect in the material being examined.
The polarization filter oriented parallel to the polarizer and placed in front of the light transducer for the polarized reflected light may be omitted. In this case a light divider is used having a reflectionztransmission ratio of approx. 0.67:0.33 so as to obtain on both receivers equal intensity of the non-polarized radiation.
3 BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a diagrammatic illustration of an arrangement according to the invention with geometrically separated transducers;
FIGURE 2 shows the material to be examined; and
FIGURE 3 shows a modified arrangement with polarization filters.
DETAILED DESCRIPTION A beam emitted by a lamp 1 is deflected by a semi-reflecting mirror 2 and is focused by a lens 3 onto a material strip 4 which is to be examined for defects. A back reflector 5 composed of reflecting material is placed underneath the material strip 4. The back reflector reflects light directed thereon in the direction of incidence and with the same vibration direction, i.e. polarization direction. Such material is well known and per se forms no part of the invention, other than its novel incorporation in the overall arrangement. The back reflector may be a foil known as Scotchlite which is a reflective foil consisting of glass pellets partially embedded in a synthetic resin material and backed with a mirror.
The light reflected by the reflector 5 is reflected in the direction of transmission and thus, after passing through the mirror 2, arrives at a photoresistance 6. The photoresistance 6 also receives a part of the light diffusely strayed in all directions by the material via lens 3. The light diffusely strayed in all directions by the material 4 also passes through the lens 3 to the photoresistancc 7. A diaphragm 9 serves for controlling the transmitted beam. The photoresistances 6 and 7 are connected in opposition in a balanced bridge and a signal will occur at output terminals of the bridge only when the photoresistances 6 and 7 are irradiated differently, i.e. when light passes through the material 4 due to the presence therein of a light passing defect 4". Thus when no defect is present in material 4 the values of photoresistances 6 and 7 will be equal and thereby the bridge will be balanced and the output equal to zero. When a defect is present in the material, the light reflected by reflector 5 will pass only to photoresistance 6 together with the dilfusely strayed light from the material 4. Thereby the values of photoresistances 6 and 7 will be unequal and the bridge will be unbalanced whereby an output signal will be produced at terminals 8.
In FIG. 2 a material 4 with dark lines 4' and a defect 4" is shown.
In the arrangement according to FIG. 3 the need for angular olfset of photoresistance 7 is avoided and the photoresistances 6 and 7 are both arranged in autocollimation with the light source and thereby scan the same region of the material 4 at the same angle. In FIG. 3 a polarization filter 10 is arranged in front of the lamp 1. In the receiving radiation path there is further provided a semi-reflecting mirror 13 which directs approximately 50% of the reflected light to the photoresistance 7 while the remainder arrives at the photoresistance 6. In front of the photoresistance 6 is an analyzer 12 having a transmission direction which corresponds to that of the polarizer 10. Accordingly, approximately half of the radiation diffusely reflected by the material 4 to the lens 3 is received by the photoelectric transducer 6 which, in case of a defect 4", also receives that part of the radiation reflected polarized by the reflector 5. The transducer 7 has an analyzer 11 positioned in front thereof which polarizes light in a direction perpendicular to that of analyzer 12 and receives about 50% only of the radiation diffusely reflected by the material 4 in the direction of the lens 3. The light reflected by reflector 5 is polarized in a direction perpendicular to analyzer 11 and therefore this light is blocked by the analyzer and is unable to reach photoelectric transducer 7.
The analyzer 12 may be omitted if the reflectionztransmission ratio of the radiation divider 13 is selected to be approximately 0.67:0.33. In such case, the light intensity on the transducers 6 and 7 will be equal.
What is claimed is:
1. Apparatus for detecting defects in woven material strips comprising photoelectric scanning means for examing material for defects, and a back reflector disposed beneath the material to be examined, said back reflector reflecting the light directed thereon in the direction of incidence and with the same polarization direction as the incident light, said scanning means comprising a light source for irradiating the material to be examined, first and second light sensitive means positioned to receive light reflected from the irradiated material and coupled together in opposition to provide zero output under the influence of light reflected diffusely from said material, only one of said light sensitive means being responsive to light reflected from the back reflector due to the existence in the material of a light passing defect, said other light sensitive means being responsive to light diffused by the material, whereby the coupled first and second light sensitive means will produce an output signal when said defect is irradiated.
2. Apparatus as claimed in claim 1 wherein said first and second light sensitive means respond with equal sensitivity to the light reflected from the material.
3. Apparatus as claimed in claim 1 wherein said one of the light sensitive means is arranged in autocollimation with respect to the light source to receive the ligt reflected by the back reflector.
4. Apparatus as claimed in claim 3 wherein said other of said light sensitive means is angularly offset from said one light sensitive means to be out of the path of the light reflected by the back reflector.
5. Apparatus as claimed in claim 1 wherein said first and second light sensitive means are in autocollimation with the light source, said apparatus comprising means for polarizing the light emitted by said source before the light irradiates the material to be examined, and a polarizing device arranged in the path of the light beam to said other of the light sensitive means, said polarizing device having a polarizing direction perpendicular to the direction of polarization of said polarizing means whereby the reflected light from the back reflector will be prevented from reaching said other light sensitive means.
6. Apparatus as claimed in claim 5 comprising a second polarizing device, the latter being arranged in the path of light reflected to said one light sensitive means, said second polarizing device having the same polarizing direction as said polarizing means.
7. Apparatus as claimed in claim 5 comprising a radiation divider in the path of the reflected light for transmitting a portion of the light to the first light sensitive means and for reflecting a portion of the light to the second light sensitive means.
8. Apparatus as claimed in claim 7 wherein said second light sensitive means is said other of the sensitive means and the radiation divider has a reflectionztransmission ratio of approximately 9. Apparatus as claimed in claim 1 comprising a bridge connecting said first and second light sensitive means and having a balanced state when the first and second light sensitive means receive only diffusely reflected light from the irradiated material.
10. Apparatus as claimed in claim 9 wherein each said light sensitive means is a photoelectronic transducer.
References Cited WALTER STOLWEIN, Primary Examiner Us. 01. X.R. 356-200, 23